Determinants of CREB degradation and KChIP2 gene transcription in cardiac memory

BACKGROUND Left ventricular pacing (LVP) to induce cardiac memory (CM) in dogs results in a decreased transient outward K current (I-to) and reduced mRNA and protein of the Ito channel accessory subunit, KChIP2. The KChIP2 decrease is attributed to a decrease in its transcription factor, cyclic adenosine monophosphate response element binding protein (CREB). OBJECTIVE This study sought to determine the mechanisms responsible for the CREB decrease that is initiated by LVP. METHODS CM was quantified as T-wave vector displacement in 18 LVP dogs. In 5 dogs, angiotensin II receptor blocker, saralasin, was infused before and during pacing. In 3 dogs, proteasomal inhibitor, lactacystin, was injected into the left anterior descending artery before LVP. Epicardial biopsy samples were taken before and after LVP. Neonatal rat cardiomyocytes (NRCM) were incubated with H2O2 (50 mu mol/l) for 1 hour with or without lactacystin. RESULTS LVP significantly displaced the T-wave vector and was associated with increased lipid peroxidation and increased tissue angiotensin II levels. Saralasin prevented T-vector displacement and lipid peroxidation. CREB was significantly decreased after 2 hours of LVP and was comparably decreased in H2O2-treated NRCM. Lactacystin inhibited the CREB decrease in LVP dogs and H2O2-treated NRCM. LVP and H2O2 both induced CREB ubiquitination, and the H2O2-induced CREB decrease was prevented by knocking down ubiquitin. CONCLUSION LVP initiates myocardial angiotensin II production and reactive oxygen species synthesis, leading to CREB ubiquitination and its proteasomal degradation. This sequence of events would explain the pacing-induced reduction in KChIP2, and contribute to altered repolarization and the T-wave changes of cardiac memory.